Processes affecting pollution fluxes during the coastal erosion of legacy colliery wastes

The fate and behaviour of legacy coastal waste deposits are of concern due to the potential for future climate change to increase contaminant fluxes from these sources. During the 20th century 39-47 million m3 of pyritic colliery spoil was dumped onto beaches in County Durham, Northeast England. Distribution of this waste along the coastline produced a 4-6 m high compacted colliery spoil terrace, which extended around 200 m seaward of original shoreline. After the cessation of tipping between 1974 and 1993, ongoing erosion of spoil terraces at these sites has become a source of several potentially toxic elements (PTEs) to coastal ecosystems. The compacted spoil material had an in-situ bulk density of 1.8 (1.2-2.0) t m−3 and moisture content of 21 (11-31) wt. % and contained potentially problematic mean concentrations of As (142 mg kg−1), Ba (1300 mg kg−1), Hg (0.63 mg kg−1), Pb (154 mg kg−1), and Zn (111 mg kg−1). Leaching tests showed that the remobilisation of these PTEs was low across a range of pH, indicating particulate dispersion as the most important pollutant transport mechanism at these sites. However, particle winnowing and density separation occur during spoil erosion. Fe oxide and pyrite grains from the spoil were preferentially retained in adjacent fine sand intertidal sediments, which as a result, also contained high concentrations of PTEs. Analysis of beach volume changes between 2010 and 2023 revealed that approximately 500,000 m3 of spoil was removed by coastal erosion in this period. This equates to median annual fluxes of As (6.6 t yr−1), Hg (0.027 t yr−1), and Pb (4.6 t yr−1), and Zn (4.5 t yr−1), which are not trivial when compared to annual fluxes from permitted UK industrial discharges or metal mine discharges in England and Wales. The high flux of As is particularly of concern due to the lack of other significant local sources of this metalloid in Northeast England. Although >95 % of the originally tipped spoil has already been dispersed offshore, approximately 1 million m3 of spoil is currently still present in beach deposits, which will remain a significant source of PTEs to coastal environments for several more decades.